The present invention relates to clay-free aqueous well drilling and servicing fluids, methods of preparation thereof, and methods of drilling or servicing a well therewith.
The use of fluids for conducting various operations in the boreholes of oil and gas wells which contact a hydrocarbon-containing subterranean formation are well known. Thus, drill-in fluids are utilized when initially drilling into potential hydrocarbon producing formations. Completion fluids are utilized when conducting various completion operations in the hydrocarbon-containing formations. Workover fluids are utilized when conducting workover operations of previously completed wells.
It is important that the fluids which contact hydrocarbon-containing formations are formulated such that there is a minimum penetration of fluid, both the aqueous phase and the solid phase, into the formation. Thus, the present state-of-the-art fluids generally comprise a “water soluble” polymer, preferably a biopolymer such as xanthan gum or scleroglucan gum, starch derivatives for fluid loss control, and water soluble or acid soluble bridging agents to form a thin filter cake which forms a protective seal of the formation. See for example the following U.S. Patents, incorporated herein by reference: Mondshine U.S. Pat. No. 4,620,596; Dobson, Jr. et al. U.S. Pat. No. 4,822,500; Dobson, Jr. et al. U.S. Pat. No. 5,629,271; Dobson, Jr. et al. U.S. Pat. No. 5,641,728; Dobson, Jr. et al. U.S. Pat. No. 5,728,652; and Dobson, Jr. et al. U.S. Pat. No. 5,804,535. A recent development is a biopolymer-free fluid which utilizes a unique amylopectin starch derivative for both viscosity development and fluid loss control as set forth in Dobson, Jr. et al. U.S. Pat. No. 6,391,830.
After the well has been drilled and completed, it is necessary to remove the filter cake from the surface of the formation allowing the hydrocarbons therein to flow to the wellbore for production. This is generally aided by contacting the filter cake with various washes/soak solutions in which the components of the filter cake are soluble, most generally acidic aqueous fluids. See, for example, the following U.S. patents, incorporated herein by reference: Mondshine et al. U.S. Pat. No. 5,238,065; Dobson, Jr. et al. U.S. Pat. No. 5,607,905; Dobson, Jr. et al. U.S. Pat. No. 5,783,527; and Dobson, Jr. et al. U.S. Pat. No. 5,783,526.
As indicated in Mondshine U.S. Pat. No. 4,620,596, sparingly soluble borates have been utilized as bridging agents in well drilling and servicing fluids. However, one problem with their use in biopolymer-containing fluids is the crosslinking of the biopolymers that occurs when the borate anion reacts with the biopolymers. Thus, there is a need for another bridging agent that is sparingly soluble in water/aqueous systems and is soluble in acidic solutions.
Magnesium oxide and other particulate solids which are substantially insoluble in water but which are substantially soluble in aqueous ammonium salt solutions are disclosed for use as bridging agents in Todd et al. U.S. Pat. No. 6,422,314. Also disclosed therein in the well drilling and servicing fluids containing the magnesium oxide bridging agent is the use of certain water soluble salts, including sodium citrate, to increase the density of the fluids.
Powdered magnesium oxide is utilized in the art as an alkalinity control additive for biopolymer-containing fluids as exemplified by the U.S. patents referenced hereinbefore.
The magnesium oxide as referenced in Dobson, Jr. et al. U.S. Pat. No. 5,514,644, incorporated herein by reference, has an Activity Index less than about 100 seconds, most preferably less than about 50 seconds.
Disclosed in International patent application Serial No. PCT/US04/33237 filed Oct. 8, 2004 is the use of calcined magnesia which has an Activity Index greater than about 800 seconds as a bridging agent in polymer-containing well drilling and servicing fluids.
The calcined magnesia provides biopolymer-containing well drilling and servicing fluids which do not gel on thermal aging at temperatures at which the biopolymer does not decompose and which utilizes the particulate, sized magnesia particles as a bridging agent to form the required thin filter cake to limit fluid invasion into the hydrocarbon-containing formation contacted by the fluid.
The present invention pertains to stable well drilling and servicing fluids which provide a filter cake that is partially water soluble, soluble in ammonium salt solutions, and substantially acid soluble for improved removal from the sides of the borehole/face of the hydrocarbon-containing formations on which the filter cake is deposited, and a method of increasing the low shear rate viscosity thereof.